Several methods have been developed to study the muscle elasticity, such as indentation, sonoelastography, transient elastography, shear-wave dispersion ultrasound vibrometry (SDUV) and magnetic resonance elastography (MRE). Each has unique features and limitations in
studies. On the other hand, although it has proved that muscle elasticity is correlated to the non-fatiguing muscle contraction, few studies have been reported to measure the muscle elastic modulus at different muscle activation levels expressed as percentage of maximum voluntary contraction (MVC), especially at the levels larger than 40%. In this study, using a new ultrasound-based method, we assessed the shear modulus of vastus intermedius, one muscle of the quadriceps femoris, at different isometric step contraction levels from 0% to 100% MVC. Seven young healthy male subjects volunteered to participate and the experiments were repeated at two knee joint angles, 90º and 120º. Our results demonstrated that the elasticity of vastus intermedius in the longitudinal direction of muscle fibers is positively correlated to the isometric contraction levels at both joint angles (90º: Pr = 0.957, p<0.001; 120º: Pr = 0.982, p<0.001) and there is a quadratic relationship between them as determined by regression analysis (90º: R
= 0.986, p<0.001; 120º: R
= 0.965, p<0.001). However, deviations at the levels higher than 60% MVC became larger. Further studies are required to investigate whether this larger deviation is the inherent property of the muscle or caused by the limitation of the measurement system. The study may advance the further understanding of the muscle function and provide us a unique tool for muscle assessment in clinic and strength training.